The present invention relates to a drive method for a switching power converter such as a DC/DC converter and to an apparatus using this method.
In general, high efficiency, high-speed switching and compactness are required of switching power converters. One approach to satisfy the last of these demands is to provide a low-power driver for the converter main switch. One way of achieving such a low-power driver is to reduce the drive current supplied to the main switch by using Darlington-connected bipolar transistors in the main switch. Darlington-connected transistors are equivalent to a transistor having a large current amplification factor (hFE). Therefore, the power required to drive such transistors is less than that required to drive a single low hFE power transistor. However, in actual circuit operation, the effective collector-emitter saturation voltage VCE(sat) of Darlington-connected transistors is twice or more that of a single power transistor. Thus, power loss incurred to the Darlington-connected transistors in the ON period of the main switch is twice or more that for a single power transistor. This is contradictory to the demand for high efficiency.
In Darlington-connected transistors, two or more transistors are sequentially (or time-serially) turned on and off. Therefore, when Darlington-connected transistors are used in the main switch, switching speed is lower than that obtained with a single transistor. This is contradictory to the demand for high-speed switching.
Thus, the two problems of increased power loss and reduced switching speed are encountered when Darlington-connected transistors are used in the main switch of a power converter.
As another means of drivng the main switch of a power converter at low power, an MOS power FET is used in the main switch. The gate input impedance of an MOSFET is much higher than the base input impedance of a bipolar transistor. Therefore, power required for driving an MOSFET is much smaller than that required for driving a bipolar transistor. In addition, MOS power FETs generally have higher switching speeds that bipolar power transistors and can realize high-speed switching at 100 kHz or higher.
When an MOS power FET is used for large current (10 A or more) switching, power loss increases significantly. This is because the drain-source voltage drop in the ON period of an MOSFET, in which a drain current flows, becomes 3 times or more the VCE(sat) of a bipolar transistor, in which a collector current of the same magnitude as the above drain current flows. This is again contradictory to the demand for high efficiency.
Thus, when an MOSFET is used in the main switch of a power converter to reduce its drive power, the problem of increased power loss is encountered.
The following reference disclosed one prior art solution to the problems encountered when Darlington-connected transistors (low efficiency, low-speed switching) or MOSFETs (low efficiency) are used:
"A NEW IMPROVED AND SIMPLIFIED PROPORTIONAL BASE DRIVE CIRCUIT", Rudy Sevens, Intersil, Inc., Proceedings of Powercon 6, May, 1979, pp. B2-1 to B2-12. In the power converter disclosed in the Rudy document, a positive current feedback is effected, through a current transformer using a saturable reactor, by a bipolar power transistor which serves as a main switch. In the Rudy power converter, a switching device having a high power loss such as Darlington-connected transistors or MOSFET is not used. Instead, a single bipolar transistor is used to constitute the main switch. Therefore, power conversion efficiency is high. In addition, since the positive feedback operation in combined with ON/OFF operation of the single bipolar transistor, switching speed is also high.
In the Rudy power converter, however, the main switch is compulsorily turned off by an output from the drive circuit. Therefore, the main switch driver requires high power to reliably turn off the main switch.
In other words, the Rudy power converter cannot satisfy the low-power consumption requirement for the main switch driver, and the overall power converter cannot be rendered compact.
All disclosures of the Rudy document are incorporated in this application.